gem install pg error, Mac Lion - postgresql

Question

I'm on Mac OS 10.7.4, and I have the latest version of Xcode and the Command Line Tools installed.
I'm getting an error (shown below) when I try to "gem install pg":
gem install pg
Building native extensions. This could take a while...
ERROR: Error installing pg:
ERROR: Failed to build gem native extension.
/Users/rdp/.rvm/rubies/ruby-1.9.3-p125/bin/ruby extconf.rb
checking for pg_config... yes
Using config values from /usr/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/rdp/.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
--without-pg-config
--with-pg_config
--without-pg_config
/Users/rdp/.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/rdp/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:506:in `try_cpp'
from /Users/rdp/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:970:in `block in find_header'
from /Users/rdp/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:790:in `block in checking_for'
from /Users/rdp/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:284:in `block (2 levels) in postpone'
from /Users/rdp/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:254:in `open'
from /Users/rdp/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:284:in `block in postpone'
from /Users/rdp/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:254:in `open'
from /Users/rdp/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:280:in `postpone'
from /Users/rdp/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:789:in `checking_for'
from /Users/rdp/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/1.9.1/mkmf.rb:969:in `find_header'
from extconf.rb:41:in `<main>'
Gem files will remain installed in /Users/rdp/.rvm/gems/ruby-1.9.3-p125/gems/pg-0.14.0 for inspection.
Results logged to /Users/rdp/.rvm/gems/ruby-1.9.3-p125/gems/pg-0.14.0/ext/gem_make.out
Here's the content of my mkmf.log file:
find_executable: checking for pg_config... -------------------- yes
--------------------
"/usr/bin/gcc-4.2 -o conftest -I/Users/rdp/.rvm/rubies/ruby-1.9.3-p125/include/ruby-1.9.1/x86_64-darwin11.3.0 -I/Users/rdp/.rvm/rubies/ruby-1.9.3-p125/include/ruby-1.9.1/ruby/backward -I/Us\
ers/rdp/.rvm/rubies/ruby-1.9.3-p125/include/ruby-1.9.1 -I. -I/Users/rdp/.rvm/usr/include -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -I/usr/include -O3 -ggdb -Wextra -Wno-unused-parameter -Wno-pa\
rentheses -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/rdp/.rvm/rubies/ruby-1.9.3-p125/lib -L/Users/rdp/.rvm/usr/lib -L. -L/usr/lib -lruby.1.9.1 -lpthread -ldl -lobjc "
checked program was:
/* begin */
1: #include "ruby.h"
2:
3: int main() {return 0;}
/* end */

Answer 1

Start XCode, go to the preferences, go to Download and install the Command line tools