Do not pass --no-cpp_precomp to compilers on Mac OS X

This option is  obsolete since Apple gcc 3.0, by  dropping support for
gcc 2.95.x we no longer have to pass this option.  By not passing this
option we sidestep  an issue for Mac  OS X when a GNU  gcc compiler is
used to build instead Apple gcc or clang.

README.developer

@@ -595,7 +595,6 @@ Keep in mind Hamlib has to run under:
 
 Hamlib should also compile with the following common compilers:
 
-    * gcc-2.9x  (most likely deprecated)
     * gcc-3.0 and gcc-3.2+  (nearly deprecated?)
     * gcc-4.x and newer
     * in shared and static

configure.ac

@@ -271,11 +271,6 @@ AS_CASE(["$host_os"],
         AC_SUBST([AM_LDFLAGS])],
 
     [darwin* | rhapsody*], [
-        # Trick from http://fink.sourceforge.net/doc/porting/
-        # TODO: check the compiler actually does support these options
-        AM_CFLAGS="${AM_CFLAGS} -no-cpp-precomp"
-        AM_CXXFLAGS="${AM_CXXFLAGS} -no-cpp-precomp"
-
         # Tell the OS X linker to allocate enough space inside the
         # libhamlib.X.dylib shared object for install_name_tool(1) to
         # work. This is useful when including hamlib in an app bundle.