Partially revert "Build: Fix build of library dependencies on Linux aarch64"

This reverts the Flex-related parts of commit
rBef268c78933079137288e326704431432adf9ad9, as those caused a build
error on CentOS 7 (which is used for the precompiled Linux libraries).

CentOS 7 only has Automake 1.13, whereas after this commit version 1.15
seems to be required.

Since in its patch description (D15319) it's mentioned that this
"probably doesn't warrant changing", and it's actually blocking the
build of the precompiled libraries for Blender 3.3 now, I'll revert the
Flex-related part of the commit.

build_files/build_environment/cmake/flex.cmake

@@ -5,8 +5,6 @@ ExternalProject_Add(external_flex
   URL_HASH ${FLEX_HASH_TYPE}=${FLEX_HASH}
   DOWNLOAD_DIR ${DOWNLOAD_DIR}
   PREFIX ${BUILD_DIR}/flex
-  # This patch fixes build with some versions of glibc (https://github.com/westes/flex/commit/24fd0551333e7eded87b64dd36062da3df2f6380)
-  PATCH_COMMAND ${PATCH_CMD} -d ${BUILD_DIR}/flex/src/external_flex < ${PATCH_DIR}/flex.diff
   CONFIGURE_COMMAND ${CONFIGURE_ENV} && cd ${BUILD_DIR}/flex/src/external_flex/ && ${CONFIGURE_COMMAND} --prefix=${LIBDIR}/flex
   BUILD_COMMAND ${CONFIGURE_ENV} && cd ${BUILD_DIR}/flex/src/external_flex/ && make -j${MAKE_THREADS}
   INSTALL_COMMAND ${CONFIGURE_ENV} && cd ${BUILD_DIR}/flex/src/external_flex/ && make install

build_files/build_environment/patches/flex.diff

@@ -1,15 +0,0 @@
-diff --git a/configure.ac b/configure.ac
-index c6f12d644..3c977a4e3 100644
---- a/configure.ac
-+++ b/configure.ac
-@@ -25,8 +25,10 @@
- # autoconf requirements and initialization
- 
- AC_INIT([the fast lexical analyser generator],[2.6.4],[flex-help@lists.sourceforge.net],[flex])
-+AC_PREREQ([2.60])
- AC_CONFIG_SRCDIR([src/scan.l])
- AC_CONFIG_AUX_DIR([build-aux])
-+AC_USE_SYSTEM_EXTENSIONS
- LT_INIT
- AM_INIT_AUTOMAKE([1.15 -Wno-portability foreign std-options dist-lzip parallel-tests subdir-objects])
- AC_CONFIG_HEADER([src/config.h])