这个问题有很多变体，但似乎没有一个可以解决我的问题。
我正在运行 OSX Lion (10.7.3)。安装了最新的 XCode。
我已经使用 postgresapp.com 的 Postgres.app 包安装了 Postgres。
但是当我尝试安装 pg gem 时，我遇到了失败：
$ gem install pg -- --with-pg-config=/Applications/Postgres.app/Contents/MacOS/bin/pg_config
Building native extensions. This could take a while...
ERROR: Error installing pg:
ERROR: Failed to build gem native extension.
/Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/bin/ruby extconf.rb --with-pg-config=/Applications/Postgres.app/Contents/MacOS/bin/pg_config
Using config values from /Applications/Postgres.app/Contents/MacOS/bin/pg_config
checking for libpq-fe.h... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers. Check the mkmf.log file for more
details. You may need configuration options.
Provided configuration options:
--with-opt-dir
--with-opt-include
--without-opt-include=${opt-dir}/include
--with-opt-lib
--without-opt-lib=${opt-dir}/lib
--with-make-prog
--without-make-prog
--srcdir=.
--curdir
--ruby=/Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/bin/ruby
--with-pg
--without-pg
--with-pg-dir
--without-pg-dir
--with-pg-include
--without-pg-include=${pg-dir}/include
--with-pg-lib
--without-pg-lib=${pg-dir}/lib
--with-pg-config
/Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:381:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
from /Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:506:in `try_cpp'
from /Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:970:in `block in find_header'
from /Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:790:in `block in checking_for'
from /Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:284:in `block (2 levels) in postpone'
from /Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:254:in `open'
from /Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:284:in `block in postpone'
from /Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:254:in `open'
from /Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:280:in `postpone'
from /Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:789:in `checking_for'
from /Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:969:in `find_header'
from extconf.rb:40:in `<main>'
Gem files will remain installed in /Users/disaacs/.rvm/gems/ruby-1.9.3-p125/gems/pg-0.13.2 for inspection.
Results logged to /Users/disaacs/.rvm/gems/ruby-1.9.3-p125/gems/pg-0.13.2/ext/gem_make.out
该/Applications/Postgres.app/Contents/MacOS/bin/pg_config命令的输出是：
$ /Applications/Postgres.app/Contents/MacOS/bin/pg_config
BINDIR = /Applications/Postgres.app/Contents/MacOS/bin
DOCDIR = /Applications/Postgres.app/Contents/MacOS/share/doc
HTMLDIR = /Applications/Postgres.app/Contents/MacOS/share/doc
INCLUDEDIR = /Applications/Postgres.app/Contents/MacOS/include
PKGINCLUDEDIR = /Applications/Postgres.app/Contents/MacOS/include
INCLUDEDIR-SERVER = /Applications/Postgres.app/Contents/MacOS/include/server
LIBDIR = /Applications/Postgres.app/Contents/MacOS/lib
PKGLIBDIR = /Applications/Postgres.app/Contents/MacOS/lib
LOCALEDIR = /Applications/Postgres.app/Contents/MacOS/share/locale
MANDIR = /Applications/Postgres.app/Contents/MacOS/share/man
SHAREDIR = /Applications/Postgres.app/Contents/MacOS/share
SYSCONFDIR = /Applications/Postgres.app/Contents/MacOS/etc
PGXS = /Applications/Postgres.app/Contents/MacOS/lib/pgxs/src/makefiles/pgxs.mk
CONFIGURE = '--prefix=/Users/mattt/Code/heroku/Postgres/Postgres/Vendor/postgres' '--enable-thread-safety' '--with-openssl' '--with-gssapi' '--with-bonjour' '--with-krb5' '--with-libxml' '--with-libxslt' '--with-ossp-uuid' '--with-perl' '--with-python'
CC = gcc
CPPFLAGS = -I/usr/include/libxml2
CFLAGS = -O2 -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wformat-security -fno-strict-aliasing -fwrapv
CFLAGS_SL =
LDFLAGS = -Wl,-dead_strip_dylibs
LDFLAGS_EX =
LDFLAGS_SL =
LIBS = -lpgport -lxslt -lxml2 -lssl -lcrypto -lgssapi_krb5 -lz -lreadline -lm
VERSION = PostgreSQL 9.1.3
所有这些设置似乎都是正确的，除了 CONFIGURE 变量（它似乎是指其他一些计算机——我没有名为 mattt 的用户）。
mkmf.log 文件的内容是
"/usr/bin/gcc-4.2 -o conftest -I/Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/include/ruby-1.9.1/x86_64-darwin10.8.0 -I/Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/include/ruby-1.9.1/ruby/backward -I/Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/include/ruby-1.9.1 -I. -I/Users/disaacs/.rvm/usr/include -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -I/Applications/Postgres.app/Contents/MacOS/include -O3 -ggdb -Wextra -Wno-unused-parameter -Wno-parentheses -Wno-long-long -Wno-missing-field-initializers -Wpointer-arith -Wwrite-strings -Wdeclaration-after-statement -Wshorten-64-to-32 -Wimplicit-function-declaration -fno-common -pipe conftest.c -L. -L/Users/disaacs/.rvm/rubies/ruby-1.9.3-p125/lib -L/Users/disaacs/.rvm/usr/lib -L. -L/usr/local/lib -L/Applications/Postgres.app/Contents/MacOS/lib -lruby.1.9.1 -lpthread -ldl -lobjc "
checked program was:
/* begin */
1: #include "ruby.h"
2:
3: int main() {return 0;}
/* end */
任何建议，将不胜感激。
我查看了 extconf.rb 文件，经过一些调试，我发现它是一个调用
find_header ('libpq-fe.h')
那是失败的。不是因为 libpq-fe.h 不存在，而是因为find_header方法本身失败了。我尝试使用 irb 运行 find_header，得到了同样的错误：
$ irb
1.9.3-p125 :001 > require 'mkmf'
=> true
1.9.3-p125 :002 > find_header 'libpq-fe.h'
checking for libpq-fe.h... RuntimeError: The compiler failed to generate an executable file.
You have to install development tools first.
编辑：
发现我的电脑一定比我想象的还要糟糕。我正在为另一个项目进行捆绑安装，安装 sqlite3 gem 时遇到了非常相似的失败。
checking for sqlite3.h... *** extconf.rb failed ***
过去我从来没有遇到过 sqlite3 gem 的问题。
