If a machine has no PCRE, grep fails to build grep on it du to
-Werror=unused-variable with --enable-gcc-warnings.  I found it on
Fedora 21 (GCC 4.9.2).

$ ./configure --enable-gcc-warnings

$ make
make  all-recursive
make[1]: Entering directory '/usr/src/gnu/grep-2.21'
Making all in po
make[2]: Entering directory '/usr/src/gnu/grep-2.21/po'
make[2]: Nothing to be done for 'all'.
make[2]: Leaving directory '/usr/src/gnu/grep-2.21/po'
Making all in lib
make[2]: Entering directory '/usr/src/gnu/grep-2.21/lib'
make  all-am
make[3]: Entering directory '/usr/src/gnu/grep-2.21/lib'
make[3]: Nothing to be done for 'all-am'.
make[3]: Leaving directory '/usr/src/gnu/grep-2.21/lib'
make[2]: Leaving directory '/usr/src/gnu/grep-2.21/lib'
Making all in doc
make[2]: Entering directory '/usr/src/gnu/grep-2.21/doc'
make[2]: Nothing to be done for 'all'.
make[2]: Leaving directory '/usr/src/gnu/grep-2.21/doc'
Making all in src
make[2]: Entering directory '/usr/src/gnu/grep-2.21/src'
  CC       pcresearch.o
pcresearch.c:43:12: error: 'empty_match' defined but not used [-Werror=unused-variable]
 static int empty_match[2];
            ^
cc1: all warnings being treated as errors
Makefile:1314: recipe for target 'pcresearch.o' failed
make[2]: *** [pcresearch.o] Error 1
make[2]: Leaving directory '/usr/src/gnu/grep-2.21/src'
Makefile:1254: recipe for target 'all-recursive' failed
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory '/usr/src/gnu/grep-2.21'
Makefile:1195: recipe for target 'all' failed
make: *** [all] Error 2