autoconf build: fix build on Alpine 3.11 where __attribute__((target_clones("fma","default"))) doesn't work for some reason

configure.ac

@@ -125,9 +125,29 @@ if test "$HAVE_GCC_WARNING_ZERO_AS_NULL_POINTER_CONSTANT" = "yes"; then
 AC_DEFINE_UNQUOTED(HAVE_GCC_WARNING_ZERO_AS_NULL_POINTER_CONSTANT, 1,
           [Define to 1 if the compiler supports -Wzero-as-null-pointer-constant])
 fi
-
 AC_LANG_POP([C++])
 
+dnl ---------------------------------------------------------------------------
+dnl Check if __attribute__((target_clones("fma","default"))) works
+dnl This is needed for example on Alpine Linux where for some reason, building
+dnl such tagged functions fails with 'error: the call requires 'ifunc', which is not supported by this target'
+dnl ---------------------------------------------------------------------------
+
+TARGET_CLONES_FMA_FLAGS=""
+AC_MSG_CHECKING([if target_clones_fma works])
+SAVED_CFLAGS="$CFLAGS"
+CFLAGS="$CFLAGS -Werror"
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM(
+[[
+__attribute__((target_clones("fma","default"))) void foo() {}
+]])],
+[AC_MSG_RESULT([yes])]
+[TARGET_CLONES_FMA_FLAGS="-DTARGET_CLONES_FMA_ALLOWED"],
+[AC_MSG_RESULT([no])])
+CFLAGS="$SAVED_CFLAGS"
+AC_SUBST(TARGET_CLONES_FMA_FLAGS,$TARGET_CLONES_FMA_FLAGS)
+
+
 dnl ---------------------------------------------------------------------------
 dnl Check for --enable-lto
 dnl ---------------------------------------------------------------------------

src/Makefile.am

@@ -7,7 +7,7 @@ check_PROGRAMS = geodtest
 
 AM_CPPFLAGS =	-DPROJ_LIB=\"$(pkgdatadir)\" \
 		-DMUTEX_@MUTEX_SETTING@ -I$(top_srcdir)/include @SQLITE3_CFLAGS@ @TIFF_CFLAGS@ @TIFF_ENABLED_FLAGS@ @CURL_CFLAGS@ @CURL_ENABLED_FLAGS@
-AM_CXXFLAGS =    @CXX_WFLAGS@ @FLTO_FLAG@
+AM_CXXFLAGS =    @CXX_WFLAGS@ @FLTO_FLAG@ @TARGET_CLONES_FMA_FLAGS@
 
 include_HEADERS = proj.h proj_experimental.h proj_constants.h proj_api.h geodesic.h \
 	 proj_symbol_rename.h

src/lib_proj.cmake

@@ -301,6 +301,8 @@ source_group("CMake Files" FILES CMakeLists.txt)
 # Embed PROJ_LIB data files location
 add_definitions(-DPROJ_LIB="${CMAKE_INSTALL_PREFIX}/${DATADIR}")
 
+add_definitions(-DTARGET_CLONES_FMA_ALLOWED)
+
 #################################################
 ## targets: libproj and proj_config.h
 #################################################

src/projections/tmerc.cpp

@@ -61,7 +61,7 @@ struct pj_opaque_exact {
 // mechanism for that, where the compiler builds a default version, and one that
 // uses FMA. And at runtimes it figures out automatically which version can be used
 // by the current CPU. This allows to create general purpose binaries.
-#if defined(__GNUC__) && __GNUC__ >= 6 && defined(__x86_64__) && !defined(__FMA__)
+#if defined(TARGET_CLONES_FMA_ALLOWED) && defined(__GNUC__) && __GNUC__ >= 6 && defined(__x86_64__) && !defined(__FMA__)
 #define BUILD_FMA_OPTIMIZED_VERSION
 #endif