Fix some issues with generated pc files

 - Use #opt_prefix instead of #prefix or HOMEBREW_PREFIX
 - Use #version rather than hardcoded strings or @Version
 - Unify pc file method naming
 - Use String#undent

Closes Homebrew/legacy-homebrew#18695.

Formula/gd.rb

@@ -23,25 +23,25 @@ def install
     args << "--without-freetype" unless build.with? 'freetype'
     system "./configure", *args
     system "make install"
-    (lib+'pkgconfig/gdlib.pc').write pkg_file
+    (lib+'pkgconfig/gdlib.pc').write pc_file
   end
 
-  def pkg_file; <<-EOF
-prefix=#{prefix}
-exec_prefix=${prefix}
-libdir=/${exec_prefix}/lib
-includedir=/${prefix}/include
-bindir=/${prefix}/bin
-ldflags=  -L/${prefix}/lib
-
-Name: gd
-Description: A graphics library for quick creation of PNG or JPEG images
-Version: 2.0.36RC1
-Requires:
-Libs: -L${libdir} -lgd
-Libs.private: -ljpeg -lpng12 -lz -lm
-Cflags: -I${includedir}
-EOF
+  def pc_file; <<-EOS.undent
+    prefix=#{opt_prefix}
+    exec_prefix=${prefix}
+    libdir=/${exec_prefix}/lib
+    includedir=/${prefix}/include
+    bindir=/${prefix}/bin
+    ldflags=  -L/${prefix}/lib
+
+    Name: gd
+    Description: A graphics library for quick creation of PNG or JPEG images
+    Version: #{version}
+    Requires:
+    Libs: -L${libdir} -lgd
+    Libs.private: -ljpeg -lpng12 -lz -lm
+    Cflags: -I${includedir}
+    EOS
   end
 
   test do

Formula/libid3tag.rb

@@ -1,31 +1,10 @@
 require 'formula'
 
 class Libid3tag < Formula
+  homepage 'http://www.underbit.com/products/mad/'
   url 'http://downloads.sourceforge.net/project/mad/libid3tag/0.15.1b/libid3tag-0.15.1b.tar.gz'
   sha1 '4d867e8a8436e73cd7762fe0e85958e35f1e4306'
 
-  def id3tag_pc
-    return <<-EOS
-prefix=#{HOMEBREW_PREFIX}
-exec_prefix=${prefix}
-libdir=${exec_prefix}/lib
-includedir=${prefix}/include
-
-Name: id3tag
-Description: ID3 tag reading library
-Version: #{@version}
-Requires:
-Conflicts:
-Libs: -L${libdir} -lid3tag -lz
-Cflags: -I${includedir}
-    EOS
-  end
-
-
-  def homepage
-    Formula.factory('mad').homepage
-  end
-
   # Fixes serious memory leaks; see https://bugs.launchpad.net/mixxx/+bug/403586
   def patches
     base = "http://mirror.ovh.net/gentoo-portage/media-libs/libid3tag/files/0.15.1b/"
@@ -52,6 +31,22 @@ def install
     system "./configure", "--prefix=#{prefix}", "--disable-debug", "--disable-dependency-tracking"
     system "make install"
 
-    (lib+'pkgconfig/id3tag.pc').write id3tag_pc
+    (lib+'pkgconfig/id3tag.pc').write pc_file
+  end
+
+  def pc_file; <<-EOS.undent
+    prefix=#{opt_prefix}
+    exec_prefix=${prefix}
+    libdir=${exec_prefix}/lib
+    includedir=${prefix}/include
+
+    Name: id3tag
+    Description: ID3 tag reading library
+    Version: #{version}
+    Requires:
+    Conflicts:
+    Libs: -L${libdir} -lid3tag -lz
+    Cflags: -I${includedir}
+    EOS
   end
 end

Formula/libspotify.rb

@@ -17,11 +17,11 @@ def install
       ln_s "libspotify.12.1.51.dylib", "libspotify.12.dylib"
     end
 
-    (lib+'pkgconfig/libspotify.pc').write pc_content
+    (lib+'pkgconfig/libspotify.pc').write pc_file
   end
 
-  def pc_content; <<-EOS.undent
-    prefix=#{HOMEBREW_PREFIX}
+  def pc_file; <<-EOS.undent
+    prefix=#{opt_prefix}
     exec_prefix=${prefix}
     libdir=${exec_prefix}/lib
     includedir=${prefix}/include

Formula/mad.rb

@@ -9,23 +9,22 @@ def install
     fpm = MacOS.prefer_64_bit? ? '64bit': 'intel'
     system "./configure", "--disable-debugging", "--enable-fpm=#{fpm}", "--prefix=#{prefix}"
     system "make", "CFLAGS=#{ENV.cflags}", "LDFLAGS=#{ENV.ldflags}", "install"
-    (lib+'pkgconfig/mad.pc').write mad_pc
+    (lib+'pkgconfig/mad.pc').write pc_file
   end
 
-  def mad_pc
-    return <<-EOS
-prefix=#{HOMEBREW_PREFIX}
-exec_prefix=${prefix}
-libdir=${exec_prefix}/lib
-includedir=${prefix}/include
+  def pc_file; <<-EOS.undent
+    prefix=#{opt_prefix}
+    exec_prefix=${prefix}
+    libdir=${exec_prefix}/lib
+    includedir=${prefix}/include
 
-Name: mad
-Description: MPEG Audio Decoder
-Version: #{@version}
-Requires:
-Conflicts:
-Libs: -L${libdir} -lmad -lm
-Cflags: -I${includedir}
+    Name: mad
+    Description: MPEG Audio Decoder
+    Version: #{version}
+    Requires:
+    Conflicts:
+    Libs: -L${libdir} -lmad -lm
+    Cflags: -I${includedir}
     EOS
   end
 end

Formula/nss.rb

@@ -52,7 +52,7 @@ def install
       cp file, lib
     end
 
-    (lib+'pkgconfig/nss.pc').write pkg_file
+    (lib+'pkgconfig/nss.pc').write pc_file
   end
 
   test do
@@ -62,18 +62,18 @@ def install
     system "#{bin}/certutil", "-L", "-d", pwd
   end
 
-  def pkg_file; <<-EOF
-prefix=#{HOMEBREW_PREFIX}
-exec_prefix=${prefix}
-libdir=${exec_prefix}/lib
-includedir=${prefix}/include/nss
+  def pc_file; <<-EOS.undent
+    prefix=#{opt_prefix}
+    exec_prefix=${prefix}
+    libdir=${exec_prefix}/lib
+    includedir=${prefix}/include/nss
 
-Name: NSS
-Description: Mozilla Network Security Services
-Version: 3.14.1
-Requires: nspr
-Libs: -L${libdir} -lnss3 -lnssutil3 -lsmime3 -lssl3
-Cflags: -I${includedir}
-EOF
+    Name: NSS
+    Description: Mozilla Network Security Services
+    Version: #{version}
+    Requires: nspr
+    Libs: -L${libdir} -lnss3 -lnssutil3 -lsmime3 -lssl3
+    Cflags: -I${includedir}
+    EOS
   end
 end

Formula/squirrel.rb

@@ -13,11 +13,11 @@ def install
     doc.install Dir['doc/*.pdf']
     doc.install %w[etc samples]
     # See: https://github.com/mxcl/homebrew/pull/9977
-    (lib+'pkgconfig/libsquirrel.pc').write pkg_file
+    (lib+'pkgconfig/libsquirrel.pc').write pc_file
   end
 
-  def pkg_file; <<-EOS.undent
-    prefix=#{prefix}
+  def pc_file; <<-EOS.undent
+    prefix=#{opt_prefix}
     exec_prefix=${prefix}
     libdir=/${exec_prefix}/lib
     includedir=/${prefix}/include
@@ -26,7 +26,7 @@ def pkg_file; <<-EOS.undent
 
     Name: libsquirrel
     Description: squirrel library
-    Version: 3.0.2
+    Version: #{version}
 
     Requires:
     Libs: -L${libdir} -lsquirrel -lsqstdlib