regex: use REGEX_BACKEND as the cmake option name

This avoids any misunderstanding with the REGEX keyword in cmake.

CMakeLists.txt

@@ -65,7 +65,7 @@ OPTION(DEBUG_POOL			"Enable debug pool allocator"				OFF)
 OPTION(ENABLE_WERROR			"Enable compilation with -Werror"			OFF)
 OPTION(USE_BUNDLED_ZLIB    		"Use the bundled version of zlib"			OFF)
 OPTION(DEPRECATE_HARD			"Do not include deprecated functions in the library"	OFF)
-   SET(REGEX				"" CACHE STRING "Regular expression implementation. One of regcomp_l, pcre2, pcre, regcomp, or builtin.")
+   SET(REGEX_BACKEND			"" CACHE STRING "Regular expression implementation. One of regcomp_l, pcre2, pcre, regcomp, or builtin.")
 
 IF (UNIX AND NOT APPLE)
 	OPTION(ENABLE_REPRODUCIBLE_BUILDS "Enable reproducible builds"				OFF)

src/CMakeLists.txt

@@ -292,22 +292,22 @@ ENDIF()
 # Specify regular expression implementation
 FIND_PACKAGE(PCRE)
 
-IF(REGEX STREQUAL "")
+IF(REGEX_BACKEND STREQUAL "")
 	CHECK_SYMBOL_EXISTS(regcomp_l "regex.h;xlocale.h" HAVE_REGCOMP_L)
 
 	IF(HAVE_REGCOMP_L)
-		SET(REGEX "regcomp_l")
+		SET(REGEX_BACKEND "regcomp_l")
 	ELSEIF(PCRE_FOUND)
-		SET(REGEX "pcre")
+		SET(REGEX_BACKEND "pcre")
 	ELSE()
-		SET(REGEX "builtin")
+		SET(REGEX_BACKEND "builtin")
 	ENDIF()
 ENDIF()
 
-IF(REGEX STREQUAL "regcomp_l")
+IF(REGEX_BACKEND STREQUAL "regcomp_l")
 	ADD_FEATURE_INFO(regex ON "using system regcomp_l")
 	SET(GIT_REGEX_REGCOMP_L 1)
-ELSEIF(REGEX STREQUAL "pcre2")
+ELSEIF(REGEX_BACKEND STREQUAL "pcre2")
 	FIND_PACKAGE(PCRE2)
 
 	IF(NOT PCRE2_FOUND)
@@ -319,16 +319,16 @@ ELSEIF(REGEX STREQUAL "pcre2")
 
 	LIST(APPEND LIBGIT2_SYSTEM_INCLUDES ${PCRE2_INCLUDE_DIRS})
 	LIST(APPEND LIBGIT2_LIBS ${PCRE2_LIBRARIES})
-ELSEIF(REGEX STREQUAL "pcre")
+ELSEIF(REGEX_BACKEND STREQUAL "pcre")
 	ADD_FEATURE_INFO(regex ON "using system PCRE")
 	SET(GIT_REGEX_PCRE 1)
 
 	LIST(APPEND LIBGIT2_SYSTEM_INCLUDES ${PCRE_INCLUDE_DIRS})
 	LIST(APPEND LIBGIT2_LIBS ${PCRE_LIBRARIES})
-ELSEIF(REGEX STREQUAL "regcomp")
+ELSEIF(REGEX_BACKEND STREQUAL "regcomp")
 	ADD_FEATURE_INFO(regex ON "using system regcomp")
 	SET(GIT_REGEX_REGCOMP 1)
-ELSEIF(REGEX STREQUAL "builtin")
+ELSEIF(REGEX_BACKEND STREQUAL "builtin")
 	ADD_FEATURE_INFO(regex ON "using bundled PCRE")
 	SET(GIT_REGEX_BUILTIN 1)
 
@@ -336,7 +336,7 @@ ELSEIF(REGEX STREQUAL "builtin")
 	LIST(APPEND LIBGIT2_INCLUDES "${libgit2_SOURCE_DIR}/deps/pcre")
 	LIST(APPEND LIBGIT2_OBJECTS $<TARGET_OBJECTS:pcre>)
 ELSE()
-	MESSAGE(FATAL_ERROR "The REGEX option provided is not supported")
+	MESSAGE(FATAL_ERROR "The REGEX_BACKEND option provided is not supported")
 ENDIF()
 
 # Optional external dependency: http-parser