* README.git: Describe GCC and GNU make requirements

* maintMakefile: Put custom C flags into a separate variable
so they can be overridden more easily on the command line.

README.git

@@ -85,6 +85,8 @@ need to install the following extra software:
 * autopoint
 * pkg-config
 * texinfo (for makeinfo)
+* GCC (Clang's GCC emulation is not sufficient)
+* GNU make (POSIX make is not sufficient)
 
 And any tools that those utilities require (GNU m4, Perl, etc.)
 
@@ -93,10 +95,13 @@ a local installation of gnulib you can set GNULIB_SRCDIR to point to it.
 Otherwise, ./bootstrap will obtain a clone for you.
 
 Unfortunately due to issues with gnulib's getloadavg, you must have automake
-1.16.1 or above.  This version is not yet widely available in GNU/Linux
+1.16.1 or above.  This version may not yet be available through GNU/Linux
 package managers.  If you need to install from source be sure to set
 ACLOCAL_PATH to point to the pkg-config location (e.g., /usr/share/aclocal).
 
+If you want to build from Git with a non-GCC compiler, add "MAKE_CFLAGS=" to
+your make command line (or at least remove any flags your compiler does not
+support).
 
 When building from Git you must build in the source directory: "VPATH
 builds" from remote directories are not supported.  Once you've created
@@ -118,6 +123,11 @@ That is, you can just run:
 
 to build and install GNU make.
 
+NOTE! This method builds GNU make in "maintainer mode".  In this mode it will
+      be slower, possibly MUCH slower: there are various sanity checks
+      enabled.  If you want to build from Git with "maintainer mode" disabled,
+      add "MAKE_MAINTAINER_MODE=" to your make command line.
+
 
 Building From Git for Windows
 -----------------------------

maintMakefile

@@ -29,11 +29,13 @@ MAKEWEBDIR ?= $(SRCROOTDIR)/make/make-web
 # We like mondo-warnings!
 # Also force comments to be preserved.  This helps when using ccache, in
 # combination with GCC 7's implicit-fallthrough warning.
-AM_CFLAGS += -C -Wall -Wextra -Werror -Wwrite-strings -Wshadow \
+MAKE_CFLAGS := -C -Wall -Wextra -Werror -Wwrite-strings -Wshadow \
 	-Wdeclaration-after-statement -Wbad-function-cast -Wformat-security \
 	-Wtype-limits -Wunused-but-set-parameter -Wlogical-op -Wpointer-arith \
 	-Wignored-qualifiers -Wformat-signedness -Wduplicated-cond
 
+AM_CFLAGS += $(MAKE_CFLAGS)
+
 # Unfortunately the Guile headers are sometimes broken.  Convince GCC
 # to treat them as system headers so warnings are ignored.
 GUILE_CFLAGS := $(patsubst -I%,-isystem %,$(GUILE_CFLAGS))