Build: Fix Sphinx version check

Older versions of sphinx-build print the version string
in a slightly different format. This made FindSphinx.cmake fail
to extract the version number and thus behave like Sphinx
was not installed.

This commit fixes the regexp to be less restrictive on
the message format.

Signed-off-by: Vikas Katariya <Vikas.Katariya@arm.com>
Change-Id: I2dd28e4fb4d693aee91eec9a8b138a43f5f2ebb3
diff --git a/cmake/FindSphinx.cmake b/cmake/FindSphinx.cmake
index 8e3925e..d390398 100644
--- a/cmake/FindSphinx.cmake
+++ b/cmake/FindSphinx.cmake
@@ -82,7 +82,7 @@
 execute_process(COMMAND "${SPHINX_EXECUTABLE}" "--version" OUTPUT_VARIABLE _SPHINX_VERSION OUTPUT_STRIP_TRAILING_WHITESPACE ERROR_QUIET)
 #Parse output
 if(_SPHINX_VERSION)
-	if(_SPHINX_VERSION MATCHES ".*sphinx-build ([0-9.]+).*")
+	if(_SPHINX_VERSION MATCHES ".*sphinx-build[^0-9.]*([0-9.]+).*")
 		string(REGEX REPLACE ".*sphinx-build ([0-9.]+).*" "\\1" SPHINX_VERSION "${_SPHINX_VERSION}")
 	endif()
 endif()