Fix build compatibility for macOS 10.14 and below (#1280)

macOS 10.15 introduced full `std::filesystem::path` support.
Before that our use of it would cause the build to fail.
This was not caught because "-mmacosx-version-min=10.9" was only
being passed to clang++ for release builds.

This passes that flag in a new static CI test build, and introduces
a hack developed by @LIJI32 to silence the availability errors,
since we use features already available in macOS 10.9.

This means we are testing both "vanilla" building,
and building static binaries using the same configuration
as during release, which should help avoiding last-minute
surprises.

Makefile

@@ -31,7 +31,7 @@ WARNFLAGS	:= -Wall -Wno-unknown-warning-option -Wno-c99-designator
 # Overridable CXXFLAGS
 CXXFLAGS	?= -O3 -flto -DNDEBUG
 # Non-overridable CXXFLAGS
-REALCXXFLAGS	:= ${CXXFLAGS} ${WARNFLAGS} -x c++ -std=c++2a -I include \
+REALCXXFLAGS	:= ${CXXFLAGS} ${WARNFLAGS} -std=c++2a -I include \
 		   -D_POSIX_C_SOURCE=200809L -fno-exceptions -fno-rtti
 # Overridable LDFLAGS
 LDFLAGS		?=
@@ -200,7 +200,7 @@ checkdiff:
 # The rationale for some of the flags is documented in the CMakeLists.
 
 develop:
-	$Qenv ${MAKE} WARNFLAGS="${WARNFLAGS} -Werror -Wextra \
+	$Q${MAKE} WARNFLAGS="${WARNFLAGS} -Werror -Wextra \
 		-Walloc-zero -Wcast-align -Wcast-qual -Wduplicated-branches -Wduplicated-cond \
 		-Wfloat-equal -Wlogical-op -Wnull-dereference -Wshift-overflow=2 \
 		-Wstringop-overflow=4 -Wstrict-overflow=5 -Wundef -Wuninitialized -Wunused \