configure: removed -mmacosx-version-min

This option is now mostly useless. From the prespective of UltraGrid
build system (CI), more defining are the binary dependencies by
Homebrow, which doesn't allow setting older Mac version.

Also the problem is, that the macros defined in Availability.h, like
__MAC_10_7 relate to the build system, not taking into account build
target. __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ should perhaps be
used but it seems to be quite poorly defined.

Anyways, users can now provide the environment variable
$COMMON_OSX_FLAGS if really needed.

configure.ac

@@ -138,26 +138,17 @@ else
         cross_compile=no
 fi
 
-AC_MSG_CHECKING([OS kernel version (major)])
-os_version_major=`uname -r |cut -d . -f 1`
-AC_MSG_RESULT($os_version_major);
-AC_MSG_CHECKING([OS kernel version (minor)])
-os_version_minor=`uname -r |cut -d . -f 2`
-AC_MSG_RESULT($os_version_minor);
-
 if test $system = Linux; then
 	LDFLAGS="$LDFLAGS -Wl,--dynamic-list-data"
 fi
 
 if test $system = MacOSX; then
         MACOS_LEGACY=no
+        os_version_major=`uname -r |cut -d . -f 1`
         if test $os_version_major -lt 19; then # Darwin 19.0.0 is macOS 10.15
                 MACOS_LEGACY=yes
-                MIN_MACOS_VER=10.7
-        else
-                MIN_MACOS_VER=10.15
         fi
-        COMMON_OSX_FLAGS="-iframework data/redists -mmacosx-version-min=$MIN_MACOS_VER -stdlib=libc++"
+        COMMON_OSX_FLAGS="${COMMON_OSX_FLAGS:+$COMMON_OSX_FLAGS }-iframework data/redists -stdlib=libc++"
 	CFLAGS="$CFLAGS $COMMON_OSX_FLAGS"
 	CPPFLAGS="$CPPFLAGS $COMMON_OSX_FLAGS"
 	CXXFLAGS="$CXXFLAGS $COMMON_OSX_FLAGS"