The flac configury marked clang as 'not being gcc' and excluded
a lot of compiler switches, most importantly the visibility flags,
from being used with it. This was done possibly after a problem
reported at: https://github.com/erikd/libsndfile/issues/49 .
This patch does the following:
- m4/gcc_version.m4 (XIPH_GCC_VERSION): set GCC_MAJOR_VERSION and
GCC_MINOR_VERSION to 0 for non-gcc. Previously, they were left
unset.
- configure: the gcc version checks are, naturally, against non-
zero values, so, allow many compiler switches to be used with
clang without affecting real-gcc cases.
- configure: When setting CFLAGS="-O3 -funroll-loops", also set
CXXFLAGS="-O3". Prevents g++ warnings with _FORTIFY_SOURCE, i.e.:
'_FORTIFY_SOURCE requires compiling with optimization (-O)'
Tested compilation using gcc-7.3.1 and clang-5.0.2 on x86_64-linux,
and gcc-4.4.7 and clang-3.4.2 on an i686-linux. Also tested cross-
compiling for Mac OS X using clang-5.0.2.
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