Remove hard limit to c89 for clang

This seems no longer true: "Windows MSVC only supports C90 so force gnu89"

With that said, also do a revert:
 "external/libeot internally uses --std=c99, do not overrule that"

 This reverts commit 61a66b612e.

Change-Id: Id628131b4fa6b61e19da6d862d773ab36f201729
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/160454
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <thorsten.behrens@allotropia.de>

external/libeot/ExternalProject_libeot.mk

@@ -23,7 +23,6 @@ $(call gb_ExternalProject_get_state_target,libeot,build) :
 			--enable-static \
 			--disable-shared \
 			--disable-debug \
-			CFLAGS='$(filter-out -std=gnu89,$(CFLAGS))' \
 		&& $(MAKE) $(if $(verbose),V=1) \
 	)
 	$(call gb_Trace_EndRange,libeot,EXTERNAL)

solenv/gbuild/platform/com_GCC_defs.mk

@@ -186,10 +186,6 @@ gb_CXX03FLAGS := -std=gnu++98 -Wno-long-long \
     -Wno-variadic-macros -Wno-deprecated-declarations
 endif
 
-# On Windows MSVC only supports C90 so force gnu89 (especially in clang) to
-# to catch potential gnu89/C90 incompatibilities locally.
-gb_CFLAGS_COMMON += -std=gnu89
-
 ifeq ($(ENABLE_LTO),TRUE)
 ifeq ($(COM_IS_CLANG),TRUE)
 gb_LTOFLAGS := -flto=thin