_build.sh: apply mman.h hack to alpine+gcc

Did not investigate why this is needed for this combination.

_build.sh

@@ -997,17 +997,22 @@ build_single_target() {
     _BINUTILS_SUFFIX="${_CCSUFFIX}"
   fi
 
-  if [ "${_CRT}" = 'musl' ] && [ "${_DISTRO}" = 'debian' ]; then
+  if [ "${_CRT}" = 'musl' ]; then
     if [ "${_OPENSSL}" = 'quictls' ] || \
        [ "${_OPENSSL}" = 'openssl' ]; then
-      # Workaround for:
-      #   ../crypto/mem_sec.c:60:13: fatal error: linux/mman.h: No such file or directory
-      # Based on: https://github.com/openssl/openssl/issues/7207#issuecomment-880121450
-      _my_incdir='_sys_include'; rm -r -f "${_my_incdir}"; mkdir "${_my_incdir}"; _my_incdir="$(pwd)/${_my_incdir}"
-      ln -s "/usr/include/${_TRIPLETSH}/asm" "${_my_incdir}/asm"
-      ln -s "/usr/include/asm-generic"       "${_my_incdir}/asm-generic"
-      ln -s "/usr/include/linux"             "${_my_incdir}/linux"
-      _CPPFLAGS_GLOBAL="${_CPPFLAGS_GLOBAL} -isystem ${_my_incdir}"
+      mman_hack=0
+      [ "${_DISTRO}" = 'debian' ] && mman_hack=1
+      [ "${_DISTRO}" = 'alpine' ] && [ "${_CC}" = 'gcc' ] && mman_hack=1
+      if [ "${mman_hack}" = '1' ]; then
+        # Workaround for:
+        #   ../crypto/mem_sec.c:60:13: fatal error: linux/mman.h: No such file or directory
+        # Based on: https://github.com/openssl/openssl/issues/7207#issuecomment-880121450
+        _my_incdir='_sys_include'; rm -r -f "${_my_incdir}"; mkdir "${_my_incdir}"; _my_incdir="$(pwd)/${_my_incdir}"
+        ln -s "/usr/include/${_TRIPLETSH}/asm" "${_my_incdir}/asm"
+        ln -s "/usr/include/asm-generic"       "${_my_incdir}/asm-generic"
+        ln -s "/usr/include/linux"             "${_my_incdir}/linux"
+        _CPPFLAGS_GLOBAL="${_CPPFLAGS_GLOBAL} -isystem ${_my_incdir}"
+      fi
     fi
   fi